From what I've seen of ODE for doing 2D physics, there are a lot of annoyances, but It can be done. I'd assume the same would be true for Newton, possible but annoying.

I've come to view SPK as extremely simplistic. It's really only good for ragdolls and particles. If you just want particles, or objects with bounding spheres it will do that great, but that pretty simple anyway. Ragdolls also work great, but you only get a wireframe with no way to skin a model over it.

I know that aarku was interested in using SPK for his moon patrol game, but he's probably written something of his own by now. Also lightbringer and I (mostly him) were writing a game with it were you had to catch suicidal ragdolls. (I haven't forgotten yet Derek)

I'm am still working on SPK. I've completely redesigned it to focus on rigid bodies, and be more object oriented, but it's definitely still not ready. I intend to work on it this summer a lot, but you know how that goes.

If you want some simple collision detection and response, I'd suggest looking over the source and writing your own or just us SPK as is. It was a huge learning experience for me as I had never really done anything like that, maybe you could benefit similarly. If you want more robust/fancy simulation go for newton/ode, but beware that there are a some quirks when you try and use it for 2D.

SPK is pretty sweet. My only complaint is it doesn't seem to work all that well when you try to compile it on linux (I'll have to investigate this a bit more).

Scott, Exit Strategy's physics setup is still broken (from an updated version of SPK or some code tweaking or something?), I'd really like to get this resolved sometime in the next few months if you got the time. We haven't talked in a while man!